Generally in image pick-up devices having a lens system and image pick up unit, there is, for example, danger of the occurrence of shading and other impediments in picked-up images caused by peripheral light fall-off due to the lens system. To deal with such impediments, for example, the lens system may be designed using numerous lenses in order to prevent the occurrence of such impediments; but lens systems designed with numerous lenses are expensive, and often cannot readily be used in consumer equipment.
On the other hand, when signals are received according to XY coordinates as, for example, in equipment using semiconductor image pick-up devices, digital processing of the received signals can be used to correct images. Thus in the field of scanners and similar equipment, various proposals have been made of techniques for digital correction of warp, color bleeds, and other lens shading resulting, for example, from image pick-up using inexpensive lenses (Japanese Patent Laid-open No. 11-355511, Japanese Patent Laid-open No. 2000-41183).
However, implementation of these prior arts has been limited to scanners and similar fields in which a substantial amount of time can be used for correction processing, and real-time correction processing like that necessary for digital cameras is not required. On the other hand, techniques for digital correction of lens shading and similar in digital cameras have been proposed, as, for example, disclosed in Japanese Patent Laid-open No. 2000-41179.
It is thought that the above described warp, color bleeds and other lens shading in a device which uses such a lens system for image pick-up will occur as a function of the distance from the optical axis of the lens system. By correcting pixel signals resulting from image pick-up according to this distance, the above-described lens shading can be alleviated or corrected. Hence in order to perform such corrections, first it is necessary to calculate the distance of a pixel for correction from the optical axis of the lens system.
However, when calculating the distance d from the lens optical axis, in the conventional method of distance calculation, the Pythagorean theorem is used to calculate d=√x2+y2), where the distance on the X-axis between the origin O and the pixel for correction is x, and the distance on the Y-axis is y. However, this calculation method includes calculations of squares and a calculation of the expansion of a root, so that in order to realize this calculation means in hardware, circuitry on an enormous scale is necessary.
A method has also been proposed in which the screen is divided into blocks, as, for example, in FIG. 11, and correction coefficients are set for each block (Japanese Patent Laid-open No. 11-275452). However, in this method the same correction coefficient is used for [all the pixels of] a block, so that the coefficient changes suddenly at the boundaries between blocks, possibly having an adverse effect on image quality. In the attached Figure, the change has been emphasized due to circumstances of the block diagram. In reality, the luminance of the central Figure and of the background changes in step-like fashion at the boundaries of blocks, indicated by solid lines, and these changes are conspicuous.
However, in the above-described method, distances from the lens optical axis and block boundaries are all defined using coordinates; these coordinates are determined by, for example, counting the number of pixels in the image pick-up unit. However, a wide range of numbers of pixels exist in the image pick-up unit, from 100,000 pixels or less to 16,000,000 pixels. When counting the number of pixels as described above and performing corrections based on coordinates, differences in the total number of pixels result in substantial changes in the correction range.
Thus in an image pick-up unit with 790,000 pixels, which is in widespread use, the greatest distance between two arbitrary points is the 1280 pixels of the diagonal line; but in a 12,600,000 pixel image pick-up unit, the maximum distance is 5120 pixels. Consequently a distance calculating means designed for an image pick-up unit with, for example, 790,000 pixels cannot be applied without alteration to a 12,600,000 pixel image pick-up unit; hence circuits must be newly designed according to the number of pixels of the image pick-up unit to be used, thereby increasing the cost of the integrated circuits and other components.
Also, while it is possible to apply a circuit designed for an image pick-up unit with a large number of pixels to an image pick-up unit with a small number of pixels, a circuit designed on the basis of an image pick-up unit with a large number of pixels will, for example, have a large bus width, and when such a circuit is applied to an image pick-up unit with a small number of pixels, redundancy occurs in the circuit. And if, for example, correction coefficients are set to a proportion necessary for an image pick-up unit with a small number of pixels, the number of coefficients set for an image pick-up unit with a large number of pixels becomes huge, and the lookup table or other means of conversion is enormous.